1. Field of the Invention
The present invention is directed to a precipitation-hardenable alloy core rod utilized in castings to keep the cores straight and concentric with the outer surfaces of the castings, the improved casting formed with the precipitation-hardenable alloy core rod, and the associated manufacturing method.
2. Description of the Prior Art
Typically, foundries that are manufacturing castings with internal cavities place cores into the molds to create this cavity. Cores and core rods (metal reinforcement rod) have been used by foundries for hundreds of years. However, they use inexpensive materials such as cold rolled steel or other cold worked materials as core rods in their cores. The problem is that when these cold worked materials are subjected to the high temperatures of molten metal which surrounds the core, the material stress relaxes and twists, bows, or bends. This causes the core to also bow or bend which causes the core, or hollow cavity inside a casting, to not be concentric with the outside surface of the casting. This condition is known in the foundry industry as a core shift.
One type of casting that can have core shift problems is a beryllium-copper plunger tip manufactured for the die cast industry. Beryllium-copper has a melting temperature of approximately 2300 degrees Fahrenheit. A plunger tip is used to inject or push molten metal such as molten aluminum or molten magnesium into a die or mold. This process is done under intense pressures approaching 30,000 pounds per square inch. While all of this is taking place, water is flowing through the inside of the core of the plunger tip as a method of cooling the tip.
The cooling of the plunger tip, and the concentricity of the cooling chamber core is critical, because the plunger tip is designed to push the molten aluminum or magnesium through a shot sleeve, which is a steel tube. The tip is run at a very tight clearance relative to the sleeve to prevent the molten metal from getting wedged between the plunger tip and shot sleeve, which would lead to premature failure. If the concentricity is off, the plunger tip can have portions which congregate heat in the heavy sections which can lead to a thermal breakdown, heat cracking, of the beryllium copper used in form the casting. Another problem with heavy cross sections is thermal expansion of the plunger tip, which can cause the plunger tip to swell and seize in the shot sleeve.
Yet another reason for the concentricity of the plunger tip's cooling chamber (casting cavity) being very critical, is that the die cast plants (end user) that purchases the plunger tip cut the plunger tip down to smaller diameters and re-use it. After a plunger tip fails due to wear from being used in an injection machine, the die casters machine the tip down to a smaller diameter and places it back into a smaller machine. This can happen several times. The danger is that if the concentricity is off, the plunger tip can have a thin sidewall or thin front face, which could collapse from the high pressures. If this were to happen, the internal cooling water could come into contact with the molten aluminum or molten magnesium that could result in an explosion and possible injury of the machine operator.
The thought process thus far in the die cast industry for a solution to the core shift problem in castings has been to increase and use the largest diameter cold worked steel core rod as possible. The belief was that the increased diameter of the steel core rod equated to greater strength that would in turn solve the core shift problem. This approach to solving the problem failed to recognize the actual problem causing the core shift in the castings. Core shift in the castings is not caused by actual physical bending of the core rod, but by the stress relaxing of the mechanical stress that is built up in the metal of the cold worked steel core rods during their manufacturing process. The stress relaxing of the cold worked steel core rods is a more significant issue for castings that are poured at higher temperatures, such as the approximately 2300 degrees Fahrenheit at which beryllium-copper melts, for castings that are machined over time to progressively smaller dimensions with progressive thinner walls between the casting exterior of the internal cooling chamber core, such as with plunger tips, and for castings being used in a manufacturing process, such a molten metal injection molding machines, were a casting failure can have significant consequences to the manufactures, injection molding machine, and machine operator.
Additionally, there are limitations as to the diameter of the core rods (reinforcing rods) that can be used in the manufacture of a particular casting due to the physical size limitations of the parts being cast by the foundry. For example, if a foundry is trying to cast a casting with a 0.5 inch diameter hole leading to a core, it is physically impossible to place a 0.75 inch steel rod inside the core for strength.
What is needed is a core rod and associated manufacturing method that will reliably produce castings that do not have the core shift problems previously experienced in various foundry castings. In particular, a plunger tip with improved concentricity of the core within the casting is needed to allow the end user to maximize the life of the plunger tip through additional uses by machining the plunger tip to smaller diameters. Plunger tips with a more reliably uniform sidewall thickness and face thickness are needed to increase safety by minimizing the chance of the plunger tips collapsing under the high pressures of the die casting or injection molding machines.